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[uaxid.def]
Annex E (informative) Conformance with UAX #31 [uaxid]
E.2 R1 Default identifiers [uaxid.def]
E.2.1 General [uaxid.def.general]
UAX #31 specifies a default syntax for identifiers based on properties from the Unicode Character Database, UAX #44.
The general syntax is
<Identifier> := <Start> <Continue>* (<Medial> <Continue>+)*
where has the XID_Start property, has the XID_Continue property, and is a list of characters permitted between continue characters.
For C++ we add the character U+005f low line, or _, to the set of permitted characters, the set is empty, and the characters are unmodified.
In the grammar used in UAX #31, this is
<Identifier> := <Start> <Continue>*
<Start> := XID_Start + U+005f
<Continue> := <Start> + XID_Continue
This is described in the C++ grammar in [lex.name], where identifier is formed fromidentifier-start oridentifier followed by identifier-continue.
E.2.2 R1b Stable identifiers [uaxid.def.stable]
An implementation of UAX #31 may choose to guarantee that identifiers are stable across versions of the Unicode Standard.
Once a string qualifies as an identifier it does so in all future versions.
C++ does not make this guarantee, except to the extent that UAX #31 guarantees the stability of the XID_Start and XID_Continue properties.